This invention relates to modem apparatus in which a received, modulated double sideband carrier signal, representing digital bits, is converted to a stream of digital samples which are then filtered and demodulated.
P. J. Van Gerwen, N. A. M. Verhoeckx, H. A. Van Essen and F. A. M. Snijders, "Microprocessor Implementation of High-Speed Data Modems," IEEE Transactions on Communications, Vol. COM-25, No. 2, February 1977, pp. 238-250 shows a microprocessor implemented modem receiver wherein an incoming QAM signal passes through two digital phase-splitting filters prior to demodulation.
K. Watanabe, K. Inoue and Y. Sato, "A 4800 Bit/s Microprocessor Data Modem," IEEE Transactions on Communications, Vol. COM-26, No. 5, May 1978, pp. 493-498 shows a microprocessor implemented modem receiver in which a bandpass filter is followed by a digitally implemented "90.degree. phase splitter," which is in turn followed by a demodulator.
Falconer et al., U.S. Pat. No. 3,878,468 shows a modem receiver having analog phase splitting filters followed by demodulation.
Qureshi et al., U.S. Pat. No. 4,004,226 (FIG. 2), Walsh et al., U.S. Pat. No. 4,085,449 (FIG. 4) and Forney et al., U.S. Pat. No. 3,978,407 (FIG. 6) all show demodulation preceeding low pass filtering.